The invention relates to a drilling tool with a tool holder having an outer end on which a cutting insert can be mounted and an opposite inner end with a shank, and with an adjustment device for minimizing drilling-system misalignment.
Such tools equipped with cutting inserts without any possibility of adjustment are described in WO 98/07539, EP 0,088,505, EP 0,181,844, DE 27 30 418, EP 0,054,913, DE 27 51 255, or DE 44 16 040.
Standard indexable cutting plates, tool holders with plate seats for the indexable cutting plates, and even boring bars can only be manufactured within a certain tolerance range. Deviations from the ideal are additive, which leads to diameter tolerances in a drilling tool of xc2x10.15 mm. A single-cutter drilling tool changes effective hole diameter during the service life as a result of wearing of the cutting inserts. Due to wear, the radial forces are increased which limit mainly the service life of the drilling tool or that of the cutting inserts being used by over-shooting or undershooting the desired diameter tolerance. In practice it is only occasionally possible to set the entire structure of the drilling tool with the holder, the indexable cutting plate, and the spindle for the proper tolerance range right from the start in order to hold the maximum diameter tolerance to 0.3 mm over the life of the cutting inserts. In addition it is often necessary to work to a much smaller tolerance of xc2x10.1 mm.
This problem is solved by the use of a radially displaceable bit holder that is constituted by an additional tool adaptor, but this is disadvantageously very expensive.
There exists at present on the market a tool holder with an adjustment device in that the tool holder has a radially directed tapered bore in which a radially movable cone is screwed by means of which the tool holder can be deformed to a relatively small extent, allowing the correction of minor tolerance variations. The finished bore diameter can however be greater after adjustment than the desired diameter. The disadvantage of this system is that the forces brought to bear by the cone are only effective in a very small area. This leads in practice to the development at high cutting speeds of vibrations in the holder. In addition the employed cone produces an off-center distribution of mass that causes a significant eccentricity which at high cutting speeds of 500 m/min or more can have a significant effect.
German 3,500,602 describes a drilling tool of the above-described type wherein the central axis of the holder shaft can be angularly offset from the central axis of the shank by elastic deformation of the drilling tool in the plane of the cutting edges and this angular position can be fixed. This is made possible by two diametrically oppositely open slots that are formed in a common plane which extends at 90xc2x0 to the plane extending through the cutting edges of the cutting faces, perpendicular to the central axis of the drilling tool. The region of these slots is provided with an adjusting device engageable with the faces of the slots or braced on the opposite sides. With this system however there is only a point-contact adjustment and retention, risking making the workpiece too flexible especially at high cutting speeds.
It is therefore an object of the invention to improve on the above-described adjusting device to minimize tolerance deviations caused by the drilling system.
This object is achieved by the drilling tool according to claim 1.
The solution of the invention is that the slots each have an inner side formed with a cylindrical seat receiving a respective bolt of complementary size, one of the bolts being connected via a double-threaded screw extending radially of the holder with a spreading cone that is seated in a conically shaped outer recess of the tool holder. Rotation of the double-threaded screw changes the spacing to the one bolt, the spacing to the one bolt determining which side of the slot is spread or destressed. The other bolt bears via a threaded screw passing completely through a threaded hole of this bolt against the tool holder so that it effects a corresponding bracing of the tool holder in the region of the slot on this side. This above-described solution has several advantages over the prior art. First, the diametrically opposite slots form two defined regions at which the tool holder can be spread on one side and compressed on another. In addition in the compression region of the groove by the other braceable bolt a counterforce is made effective against the force of the adjustment cone and as a result tool vibrations are reduced. In addition the adjustment cone which is threaded on the double-threaded screw has a very fine pitch and thus one can very finely adjust the spreading of the slot on this side. Unlike the versions of the prior art in which the threaded cone is screwed into a preformed bore of the tool holder which is extremely prone to war as a function of how much it is spread, with this invention the spreading force is created by contact of the adjustment-cone surface and the complementary conical surface of the recess of the tool holder. The use of two bolts allows the tool holder to have maximum rigidity. In addition with the adjustment device of this invention which is formed of the two bolts, the double-threaded screw, the cone screw, and the threaded screw, there is much better weight distribution, that is the remaining mass eccentricity is minimal relative to the embodiments of the prior art. Finally the adjustment device according to the invention produces a substantially greater final tolerance in the tool system which is limited to xc2x10.1 mm, preferably xc2x10.05 mm.
Further features of the invention are seen in the dependent claims.
Thus the bolts extend parallel to each other, that is the bolts are mirror-symmetrical to an axis extending perpendicularly through the center of the tool holder. This formation makes it possible to put the conical recesses on one or the other side and the counter bolt on the opposite side. If necessary the two bolts can have the same dimension (diameter and length). The invention proposes embodiments where both opposite sides have an adjustment cone, a double-threaded screw, and a bolt as the respective adjustment devices. In this cases applying adjusting forces and counter forces can be done in the same way on both sides.
According to a further embodiment of the invention, the bolts engage flatly at least longitudinally in line contact with the respective cylindrical seats. This advantageously avoids wedging or locking of the bolts.
According to a further embodiment the tool holder has a thickened region from which the shank extends and which carries the adjustment device. In this manner one obtains the largest possible displacement angle of the tool-holder edges with cutting inserts there.
In order to ease the spreading or compression of the tool holder at the slots, the tool thickened region has, offset by 90xc2x0 to the slots (between the slots) on opposite sides and level with the slots, cutouts reducing the thickness of and weakening the thickened region or in other words the tool thickened region is reinforced on the sides with the slots.
Preferably the slot width is between 0.1 mm and 4 mm.
A diameter of the seats and of the respective bolts is greater than a width of the slots, a diameter of the one bolt and its seat being at least 20% larger than a diameter of the other (counter) bolt and its seat.
Finally according to a further embodiment of the invention a spacing between the seats holding the bolts is at least as great as the width of the tool holder at its insert-carrying end.